The gastrocnemius and the soleus combine to form the calve muscles, running between your knee and your heel.
When you exercise your calves, you are conditioning two muscles, the gastrocnemius and the soleus.
The gastrocnemius is the large visible muscle on the back of your lower leg.
The soleus lies underneath the gastrocnemius, the workhorse of your calf.
Gastrocnemius, tonic contraction obtains; but this, like the last mentioned, is not autochthonous in the muscles themselves; it is indirect and neural, and appears to be maintained reflexly.